Method of producing an adherent film of rubber on the grain surface of vegetable tanned leather



Patented Mar. 8, 1932 UNITED STATES PATENT OFFICE BRIAN HEAD, OFBELMONT, MASSACHUSETTS, ASSIGNOR TO UNITED SHOE MACHINERY CORPORATION,OF PATERSON, NEW JERSEY, A CORPORATION 01 NEW J METHOD OF PRODUCING ANADHERENT FILM OI RUEBEB ON THE GRAIN SURFACE OF VEGETABLE TAN NEDLEATHER No Drawing.

This invention relates to the production on the grain surface ofvegetable tanned leather of an adherent film of rubber to facilitate thecementing to said surface of another piece of leather or similarmaterial.

Cements having as their base rubber latex have been used in various waysin the manufacture of boots and shoes, examples of such uses beindescribed in United States Letters Patent 0. 1,77 9,368, granted October21, 1930, upon an application filed in the name of James Cavanagh. Forexample, latex has been used in the manufacture of leatherboard heelsand leatherboard bases for rubber heels to stick together the lifts ofwhich such heels and heel bases are made. When, however, attempts havebeen made to use latex as a cement in the manufacture of leather heelsor heel bases, which are made of vegetable tanned leather, it has beenfound that a firm joint is not obtained between two lifts if the surfaceof one of the lifts at the joint is a grain surface.

When the grain surface of a piece of leather is coated with latex, theaqueous dispersing medium (hereinafter referred to as water) apparentlyenters the leatherwithout carrying into it enough rubber to anchor to itthe film of rubber which remains on the surface.

' The film, therefore, does not adhere to the grain surface andconsequently any joint between that surface and another surface is veryweak. I

The interstices in the grain portion of a hide are much smaller thanthose in the flesh portion; and when,-in the process of vegetabletanning, the hide is heavily loaded with tannin, the interstices in thegrain are filled with tannin to such an extent that only very smallopenings are left. Consequently, as has been explained above, when latexis aplied to such a surface, the rubber-is strained out of the latex, asit were, and left on the surface in a practically non-adherent film.

I have found that by treating the grain surface of vegetable tannedleather with an alkali of suitable strength and applying latex to thesurface either simultaneously with or subsequent to the treatment withthe alkali,

I firmly adherent film or coat of rubber can Application filed Octobef22, 1925. Serial No. 64,282.

be deposited upon the grain surface of vegetable tanned leather, andthat consequently a firm joint can be secured between that surface andanother coated surface, whether the second surface be grain or flesh.The effect of the alkali is believed to be to dissolve or loosen thetannin and thereby open the pores of the leather to an extent sufiicientto permit the water of the latex to carry with it into the pores of theleather an amount of rubber sufiicient, when the water has been absorbedby the leather, to anchor to the leather the film of rubber.

Commercial rubber latex usually contains ammonia, but this alkali ispresent in only a small amountcommonly about .4%a concentration quiteinsufficient to produce the desired effect. By adding more ammonia tothe latexfor example, enough to raise the ammonia content to 1.2% orthereaboutsan adherent film of rubber can be produced. There are,however, objections to the use of an alkali such as ammonia because ofits volatility, by reason of the quantity of irritating and disagreeablefumes which evaporate when ammonia of the necessary strength is used,and by reason of the lowering of the ammonia. content which results fromsuch evaporation. The disadvantages involved in the use of a liquidwhich gives off irritating fumes are obvious. With regard to theresulting lowering of the ammonia content, it should be understood thatthe treated leather piecessuch, for example, as heel liftsmust beallowed to stand for an appreciable interval to give the alkali time toact sufficiently upbn the tannin, and that,.during this interval, thealkali evaporates much more rapidly than the water, its concentrationdecreases, andits action upon the tannin comes progressively less.Preferably, therefore, a non-volatile alkali is used since noobjectionable fumes are then given'ofi and, durin the interval that thecement is permitte to stand while the tannin is being acted upon, theonly evaporation which takes place is that of the water; and thisevaporation causes the concentration of the nonvolatile alkali-andconsequently its action upon the tannin-t0 increase progressively.

Strong alkalis tend to discolor leather by darkening it, and, while suchdiscoloration may not be objectionable in some cases, it may be inothers. Preferably, therefore, the alkali used should be of a strengthsuflicient to dissolve or loosen the tannin in the leather so as to openthe pores thereof but insufiicient to cause discoloration of theleather; and at the same time it should be present in suflicientquantity so that the amount used up in combining with the tannin willnot materially reduce its effectiveness; A cement which meets theseconditions should contain a non-volatile alkali in initial concentrationof not less than 1/50 equivalent per liter and of such character thatthe pH value (which is a measure of its alkalinity) can under no workingconditions rise above 11.

' Although various non-volatile alkalis, which willnot discolor leather,may be used, such, for example, as sodium arsenite or a mixture ofsodium bicarbonate and sodium carbonate, use will preferably be made ofborax, a satisfactory cement resulting when there is added to rubberlatex 20 grams (about 1/10 equivalent) of borax per liter.

The great advantage from a practical standpoint in using borax is that,because borax is a weak alkali, a considerable amount of it can be addedto the latex so as to make sure that there shall be enough present tocombine with a considerable amount of the tannin without raising the pHvalue to an extent sufiicient to cause discoloration of the leather,since even a saturated solution of borax, which is a substance freelysoluble in water, has a low pH value. It should be noted, however, thata stronger alkali which is sparingly enough soluble may be used since,owing to the sparing solubility of such an alkali, its solution will notbecome concentrated enough to raise the pH value above the limit set. Itshould, of course, be understood that the handling of latex takes placeat room temperature and that the solubility referred to above is thesolubility at 25 C. or thereabouts.

In carrying out the method of the present invention, the grain surfaceof a piece of vegetable tanned leather is coated with the cement andallowed to stand for a short time durin' which the alkali opens thepores of the leat er so that the water, as it goes into the ores,carries with it a certain amount 0 rubber; and when the water hasentered the leather, there is left on the grain surface an adherent filmof rubber. A second piece of stock, which may or may not e leather,having had one surface provided with a like adherent rubber film may bejoined firmly to the leather piece by pressing the two pieces firmlytogetlier with their films in contact.

Throughout the specification the term rubber latex has been used inorder to promote brevity. Rubber latex is essentially an aqueousdispersion of rubber in which the rubber content is commonly from 33 to38% by weight. Rubber latex may be used undiluted with water in case aparticularly strong bond is required between two articles; but for mostpurposes the latex may be diluted to a considerable extent. Just what isthe limit tex, whether diluted or not, ingredients which tendto preventcoagulation or to vary the viscosity or affect the adhesive quality. Theterm rubber latex, then, as used in the ape pended claims, is intended.to cover any cement, the basis of which is water-dispersed rubber.

Having described my invention, what I claim as new and desire to secureby Letters Patent of the United States is 1. The method of roducing anadherent film on the grain sur ace of vegetable tanned leather whichcomprises treating the natural grain surface of the leather with a weaknonvolatile alkali in a concentration suflicient to open the pores ofthe grain surface of the leather, and applying rubber latex to saidgrain surface.

2. The method of producing an adherent film on the grain surface ofvegetable tanned leather which comprises treating said surface with aweak non-volatile alkali in a concentration sufiicient to open the poresof the leather, and simultaneously applying rubber latex to saidsurface.

3. The method of producing an adherent film on the grain surface ofvegetable tanned leather which comprises treating said surface withrubber latex containing a non-volatile alkali in sufficientconcentration to open thepores of the leather and of such character thatthe pH value does not exceed 11.

4. The method of causing the grain sur- ,derstood that there may beadded to the lay face of a piece of vegetable tanned leather 0 to adhereto another piece of material which comprises, coating said grain surfacewith rubber latex containing a non-volatile alkali of such a characterthat the pH value does not exceed 11, and in sufficient concentration sothat an adherent film is formed on said grain surface, forming on asurface of the other piece an adherent film, and subsequently pressingtogether the two films.

5. The method "of producing an adherent film on. the grain surface ofvegetable tanned leather which comprises treating said sur facewithrubber latex containing borax in.

a concentration suflicient to open the pores of the leather.

6. The method of producing an adherent film on the surface of leatherwhich com prises coating said surface with rubber latex containing boraxin a concentration sufiicient to open the pores of the leather.

. 7. The method of producing an adherent film on the grain surface ofvegetable tanned leather which comprises coating the surface with rubberlatex containing a non-volatile alkali in initial concentration of notless than 1/50 equivalent per liter and of such character that the pHvalue can under no conditions rise above 11.

8. The method of causing the grain surface of a piece of vegetabletanned leather to adhere to another iece of material which comprisescoating sa1d grain surface with rubber 2o latex containing an alkali insuflicient concentration to open the pores of the leather and of suchcharacter that the pH value can under no conditions rise above 11,allowing the leather piece to stand until a dry adherent film of rubberhas formed, forming upon the other piece a dry adherent film of rubber,-and pressing together the two films.

- In testimony whereof I have signed my name to this specification.

so BRIAN MEAD.

